Build a dbl shotgun config - 2 parallel tubes of 2 cells each and add a single cell on the top. Put the single cell in the middle on top or maybe a little toward the rear. 5s a123s side-by-side are too wide to run in the older Emaxx - I have that Emaxx and I have built a pair of s-b-s a123 packs. They are way too wide to hold on the batt trays.

The footprint of a dbl shotgun type pac (with extra 5th cell on top) should fit. I have seen 6s a123s built that way - dbl shotgun with two more cells on top - a triangle config fitted to the older E and a G2 (or 3) Revo.